Code 15 - Coolant Temperature Sensor Circuit (Low temperature indicated)

Circuit Description

The coolant temperature sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage on CKT 410 to the sensor. When the engine is cold the sensor (thermistor) resistance is high, therefore the ECM will see high signal voltage.

As the engine warms, the sensor resistance becomes less, and the voltage drops. At normal engine operating temperature (85 C to 95 C) the voltage will measure about 1.5 to 2.0 volts at the ECM.

Problem Description:

Code 15 will set if:

• Ignition "ON", engine not running.

• Coolant temperature less than -30 C (-22 F) for 3 seconds.

• No Code 23

• MAT temperature indicated above -25 C (-13F).

  OR

• Engine running longer than 1 minute.

• Signal voltage indicates a coolant temperature of less than -30 C (-22 F) for 3 seconds.

Diagnostic Aids:

A Scantool reads engine temperature in degrees centigrade. After the engine is started the temperature should rise steadily to about 90 C, then stabilize when the thermostat opens. If Code 21 is also set, check CKT 452 for faulty wiring or connections. Check terminals at connector for good contact.

"Scan" Diagnostics:

Does scantool display coolant -30 C or colder?

NO YES